bug#23468: Feature request: highlight Edebug breakpoints

[Paul Pogonyshev]

Problem: when you add a debug breakpoint it works, but gives you zero
indication where it is, until execution hits it.  This is inconvenient
and also a lack compared to pretty much any other visual debugger.

Breakpoints in Elisp buffers should be highlighted with overlays
(e.g. different background color) and, probably, with a sign on the
window fringe.  It would also help to highlight conditional breakpoints
differently than unconditional ones.